With the development of small information devices, electronic components of integrated circuits and large-scale integrated circuits have been downsized at a fast pace. These electronic components include those having a large number of outer leads on side surfaces of the package, such as dual in-line packages (DIPs), quad flat packages (QFPs) and small outline packages (SOPS). These electronic components include pin-shaped connection terminals (outer leads) along their circumference, and the connection terminals and a substrate, or the like, are connected to each other via solder. Therefore, when the number of connection terminals to be provided along the circumference increases due to integration, the size of the package increases, making it difficult to improve the mounting density.
In response to these problems, mounting (BGA mounting) has been making progress in which connection terminals are provided using solder balls, which do not have small ball-shaped cores, solder-covered Cu core balls (covered metal particles), which have metal particles whose primary component is Cu (copper) as cores with the outermost surface thereof covered with a solder layer, etc. Researches are also being made for three-dimensional high-density mounting, such as package-on-packages (POPs) and multi-chip modules (MCMs), in which units to be connected are stacked together in the height direction, while connection terminals are provided using such solder balls or solder-covered Cu core balls. With such a BGA mounting or a three-dimensional high-density mounting, it is possible to significantly improve the mounting density while suppressing an increase in the size of the package.
In recent years, as the density and the performance have been increased with BGA mounting and three-dimensional high-density mounting, researches are being made for further decreasing the size of the connection terminal section, and there is a demand for further reducing the diameter of the solder ball or the solder-covered Cu core ball described above. However, simply reducing the diameter of such a solder ball or a solder-covered Cu ball decreases the area (the area of attachment) that contributes to the attachment of the connection terminal portion. When the contact area of the connection terminal portion is reduced, the electrical resistance (volume resistivity) increases, and the current density in the connection terminal portion increases even in an electric field of an equivalent level to a conventional level. Such an increase in the current density generates a void in a solder portion due to electromigration, and the void grows, thereby increasing the possibility of breaking the connection terminal portion. Moreover, the connection terminal portion exposed to a strong electric field generates heat due to the electrical resistance (volume resistivity) of itself to melt the solder portion, thereby increasing the possibility of a short-circuit failure in which the molten solder comes into contact with an adjacent connection terminal.
As one method for solving such a problem caused by an increase in the current density, focusing particularly on solder-covered Cu core balls, it has been under discussion to keep the hardness and the deformation resistance value of the metal particle to be the core each within a predetermined range to facilitate plastic deformation, and to increase the contact area by deforming the metal particle so as to flatten the metal particle when forming the connection terminal portion. Regarding metal particles suitable for such an application, Patent Document 1, for example, discloses a Cu ball (metal particle) produced by uniform droplet spray (hereinafter referred to as “UDS process”), wherein the purity by mass is 99.9% or more and 99.995% or less, the sphericity is 0.95 or more, and the Vickers hardness is 20 HV or more and 60 HV or less. The UDS process is a quench particle production process capable of efficiently producing metal particles having a high sphericity while stably suppressing variations in particle diameter, in which molten metal droplets are dripped successively and rapidly solidified. Patent Document 1 also states that an increase in purity suppresses miniaturization of the crystalline structure of the metal particles, thereby decreasing the hardness and decreasing the sphericity of the metal particles. Note that metal particles for use in connection terminals are required to have a high sphericity in order to suppress dislocation of metal particles and improve self-alignment thereof when connections are made through solder reflow, to reduce variations in the connection gap formed by metal particles, and to suppress cracking of connection terminals due to repeated shear stress.
For the decrease in sphericity due to an increase in purity, Patent Document 1 discloses ensuring a desirable sphericity by rapidly solidifying, using the UDS process, Cu balls of which the impurity (trace element) mass content is 0.005% (50 ppm) or more and the sum (Pb+Bi) of Pb (lead) and Bi (bismuth) is greater than or equal to a predetermined amount, and then desirably softening the Cu balls through an annealing process using a holding temperature of 700° C. Then, it is clearly stated that specifically, Cu balls with Cu at 99.995% or less, Pb+Bi at 27.0 ppm, a Vickers hardness of 67.5 HV and a sphericity of 0.991682 (see Comparative Example 2) were successfully turned, through an annealing process, into Cu balls with a Vickers hardness of 55.8 HV and a sphericity of 0.984764 (see Example 2). Note that it is stated that other impurities (trace elements) include Sn, Sb, Zn, As, Ag, Cd, Ni, Au, P, S, U, Th, etc., and that the method for analyzing the components of the metal particles is by the high-frequency inductively coupled plasma atomic emission spectroscopy (ICP-AES analysis).